Of course, we all know that the best solution is preëmption, but I'm curious what people think about the other two options.

By the way, I'm pretty sure both are fully standard -- neither can be said to be "wrong" as a matter of standard English usage, though particular publications may have style manuals that insist on one or the other.

I think the hyphen in "pre-emption" makes it easier to read. I recognize "pre-emption" in a single glance, but with "preemption" I have to spend a little bit of time figuring out where "preem" is going.

Of course this is only an issue the first time the word comes up, so it's probably not a huge deal.

Eugene: writing a PhD thesis on John Hart Ely and finding repeated US references to either "representation reinforcing" or even (rarely) "representationreinforcing", but almost never "representation-reinforcing".

One article (circa 1981) by Ira Lupu referred to Roe as preventing opponents of abortion from voting for "profile" candidates, which to my mind shows the value of the humble hyphen in avoiding typos.

I go for preemption, as well as cooperation. Why add a hyphen when it's just not necessary, and adds nothing to either asthetics or understanding? While adding the hyphen might aid persons unfamiliar with the word with pronunciation, that's just not enough justification to add the hyphen. Might as well delete the final e in forte so I don't have to constantly hear people say "That's just not my for-tay".

I'm with Nathan and matt: It is easier to recognize the word when it is hyphenated.

The comparison with "cooperation" is interesting, because one sees that in books that came out around the time that "coöperation" was fading. Not that long ago. The umlaut was clealry there to indicate that the "oo" should not be read as a dipthong.

But "cooperate" is such a common word, and I suspect that this is why there is no longer a felt need to indicate the syllabification of "coop." Pre-empt is not written frequently enough in general usage to qualify. In a decade, perhaps, we'll see "preempt" take over. As of now, I write "pre-emption."

(Can we also anticipate that "preemptory" will take on the meaning of "peremptory," which will then fade from the language?)

inference, preference, reference don't have hyphens, but are also prefix+root constructions (con-structions?), and probably never had hyphens.
So preemption, even though others don't like the double vowel. Written English doesn't normally use pronunciation hints (accents, graves, umlauts).

Unlike the more common evolution of noun conjunctions: "key stone" "key-stone" "keystone".

Technically speaking the double dot in coöperation is a diaeresis, although it looks the same as an umlaut. An umlaut changes the sound of the vowel,as in Schrödinger and Führer, while the diaeresis signals that the vowel it's above is to be sounded separately

The umlaut would make sense in French where I believe it would indicate that the double vowel is two sounds, not a dipthong (which is what we want here), but is there any reasonably modern precedent for using that accent in English?

Cornellian
"The umlaut would make sense in French where I believe it would indicate that the double vowel is two sounds, not a dipthong (which is what we want here), but is there any reasonably modern precedent for using that accent in English?"

Here's my solution when faced with such questions: Look up the prefix in Webster's. Generally there will be a long list of words starting with the prefix. Even if a given compound doesn't appear, there's likely to be one that is sufficiently analogous to provide an answer. Here preemption (or preempt) doesn't appear, but preembargo, preemployment, and preenrollment do. So it would appear that closed is the preference.

Of course, in this case, preempt appears as its own entry (not under pre-) in my dictionary. So I'd just go by that.

Cornellian said: "The umlaut would make sense in French where I believe it would indicate that the double vowel is two sounds, not a dipthong (which is what we want here), but is there any reasonably modern precedent for using that accent in English?"

How about naïve? Or the name Chloë? Those two are off the top of my head.

Personally, I'd like to see more of it, like in coöperation. So, I guess count me in on the "preëmption" preference.

American English has traditionally been described as hostile to hyphenated prefixes. Garner will definitely say preemption; I think even Fowler will agree and that's BrE.
So it's preemption (but use a dieresis if you're writing for the New Yorker!).

As a practicing lawyer, I would look at the opinions of the court I was arguing in. I'd do whatever they do.

If I were writing a law review article, I'd eliminate the hyphen. It's a waste of pixels, and the oddity of a hyphenated word makes it pop a little from the page. As to pronunciation, anyone reading an article that discusses preemption can pronounce it without the hyphen.

I can't believe there were this many posts on a minor linguistic topic on a Friday night.

I write "pre-emption" when I'm a) sure my audience will have difficulty pronouncing it, b) writing to people who are used to British spellings (I also shamelessly add unnecessary "u" and "e" and even sometimes swap "z" and "s"), or c) feeling like being different. "Preemption" wins on legal blogs, when I'm lazy, or when the tyranny of Microsoft Word is more important than my audience's needs. "Cooperation" is pretty much universal... but I add extra hyphens whenever I can when commenting on the BBC website, and so it becomes "co-operation" there (I do the same thing with my 8-year-old Sunday School classes, who usually get bonus lessons in math and reading while studying scripture with me.)

Also: I have no objection to hyphens. My name is hyphenated, my first cousins' names are all hyphenated, and I went to elementary school with the son of this man, whose entire family is hyphenated. Embrace semi-exotic punctuation, that's what I say; I'm already typing hyphens every day anyhow.

I thought you Americans hated hyphens on principle? That either "non self sustaining" or "nonselfsustaining" is Webster-kosher, but "non-self-sustaining" is a return to the tyranny of George III?

No tyrannical implications. It's just that during the Boston Tea Party we accidentally threw several boxes of perfectly good hyphens overboard by mistake as well, and we've been short ever since.

I practice in an area that's been throwing the word around of late, and I've seen it both ways in opinions and briefs. My sense is that use of the hyphenated version is declining but is still used my a significant minority of practitioners and judges.

In my view, prefixes that create double vowels at the junction should use a hyphen. Yes, that means I prefer co-operation to cooperation (which, at first glance, seems to refer to the action of a barrel maker).

Preemption. When adding the prefix is a novelty, writers tend to use a hyphen. As we get used to seeing the combination, the hyphen gradually disappears because a new word has been created. The umlaut is archaic. Today, it just looks cute. If you want to attract attention and slow the reader down, use it. If your goal is readability, either use the hyphen or don't, depending on whether the prefix has created a new word yet. (For the record, I like umlauts. In some languages, they're more than a guide to pronunciation. They creates different letters, which are alphabetized after z. But if I explain this, you're going to be bored, aren't you? That's because in English this is arcane knowledge.)

As several of the commentors have suggested, binary wordage usually evolves as time passes. The first order is two words. With time and extended usage, the binary evolves to a hyphenated form. As this form becomes common and easily recognized by readers the hypen is eventually dropped and the binary become a single word, i.e.;

I don't use the hyphen but for reasons that have to do with typesetting conventions. In general, where both are accepted, I drop the hyphen.

The reason is that it is incorrect to autohyphenate pre-emption when doing typesetting, so preemption is better (if it goes over the margins it can be hyphenated at pre-emption, preemp-tion, etc).

In response to LC Schreib's point, I would add though that a lot of compounds actually are inherited from Old English, where binary compounding was accepted and never hyphenated. Even in early middle English, we get compounds like Courtyard. (Note that Court comes from the Old French word which has the same meaning as yard does in Middle English, so it is a redundant compound....)